The transplants for Parkinson's disease have demonstrated that the brain retains a plasticity previously thought lost early in development. It offers the hope that a number of brain diseases can be treated with implants. High on the list for needing a treatment is Alzheimer's disease, a disorder that steals the mind of some 2.5 million to 3 million older Americans {related story, Page 8}.

Researchers know that Alzheimer's causes many changes in the brain's neocortex, the center of memory, speech and other higher brain functions. A key change is the loss of acetylcholine, a chemical messenger that carries signals from one nerve to the next.

The lack of acetylcholine "is not the cause of the disease," said England's Alan Fine, "but it causes the problems with thinking."

Normally, acetylcholine is made by nerves at the base of the brain and then pumped up to the neocortex. Researchers like Fine have begun to experiment with implanting plugs of acetylcholine- producing tissue from rat fetuses into rat models of the disease.

Once the transplant is made, "you see dramatic retention of memory," Fine said. To test the rat's memory, the researchers teach it to find a submerged platform in a certain part of a tank of water. When a rat is normal, it can quickly learn where the platform is located and will immediately swim toward it. (While rats are good swimmers, they prefer solid ground.)

An Alzheimer's rat will not remember where the platform is located and will have to swim around until it finds it randomly. But if an Alzheimer's rat is treated with a transplant, "it looks like it is remembering {the location of the platform} in a normal way," Fine said.

In experiments on aged rats that have symptoms mimicking Alzheimer's disease, Dr. Fred Gage and others at the University of California at San Diego also showed that transplants improved memory.

"What that means for Alzheimer's patients is not clear," Fine said, especially because there is other damage to the human Alzheimer's brain as well. But it is an interesting first step. Work is beginning to test these observations in monkeys. If successful, humans could follow.

Although Alzheimer's affects more people, Huntington's disease is most likely to be the next brain disorder attacked with tissue transplants. Huntington's arises when neurons in the brain's putamen and the caudate, the place where dopamine is consumed, begin to die because of a genetic defect. The result is uncontrolled, rapid movement and, ultimately, death.

Instead of simply implanting dopamine-producing cells, as physicians will in Parkinson's disease, the researchers must implant the nerve cells that use dopamine and turn its action into movement, said Dr. Patrick Brundin of the University of Lund in Sweden.

Groups in Lund and Cambridge are working with rat models to test whether transplants of fetal brain cells can reverse the disease or at least relieve the uncontrollable movements.

Huntington's will be more difficult to fix, however, because the transplanted nerve cells have to make connections with other nerve cells in the right way. Preliminary work, said Brundin, shows that the nerve cells "find their way about somehow. I don't know how."

The transplants may not be enough because these patients "also get dementia, probably related to the caudate and putamin degeneration," Brundin said.

Work also is under way to think about fixing epilepsy with transplants, said Dr. Olle Lindvall, also of the University of Lund. Much of the work in the other diseases is aimed at replacing a missing chemical -- dopamine or acetylcholine. But the seizures of epilepsy are caused by local lightening storms in the brain, uncontrolled sparks of electrical activity.

For partial complex epilepsy, which doesn't respond well to drugs, it may be possible to transplant cells that produce gamma amino buteric acid, GABA, a principal inhibitor of other neurotransmitters in the brain. By increasing the production of GABA near the initiating site of epilepsy, it may be possible to shut down the seizures.

Lindvall has implanted GABA-producing cells in the brains of rats that model one form of epilepsy. "There was a clearly significant positive effect by the {implants}," he said.

For brain injuries, such as those caused by stroke, there are times when the redundancies in the brain can take over for the lost tissue, but there are parts of the brain whose function, once lost, cannot be taken over by other parts of the brain. Researchers are beginning to test ideas for restoring the function of these damaged areas with implanted fetal cells.

Researchers at the Case Western Reserve University School of Medicine in Cleveland have begun using implants to guide the regeneration of damaged peripheral nerves. Using crushing injuries in rats, Dr. Jerry Silver's group has learned to regenerate connections between hind limbs and the spinal cord. "The potential for growth is there," Silver said, but the central nervous system seems to contain an inhibitory factor.

By using growth factors and tissue implants of a unique astroglial nerve cell that seem to be able to guide the growing nerve cells, Silver has been able to restore nerve connections in three rats.

"I am confident that we will be able to restore function," Silver said.

Researchers at the Institut de Biologie in Montpellier, France, have been using rat fetal cells to help build nerve bridges across spinal cord injuries. Ten days after the transplant, said Dr. Alain Privat, fibers from the transplanted cells could be seen growing across the break and reaching their target cells on the other side. And, Private said, there is some evidence that the transplanted cells restored function in a few of the animals.

Diabetes insipidus is another disorder that may be treated with transplants. It is a neuroendocrine disorder in which head injuries can destroy the hypothalamus's ability to make vasopressin, a hormone into the blood that regulates the kidneys' ability to concentrate salt in the urine.

By transplanting rat fetal hypothalamus, Dr. Don Gash at the University of Rochester has been able to produce complete recovery in half of the treated rats.

One group is actually learning to grow what is essentially a new brain. At the Institute for Basic Research in Developmental Disabilities on Staten Island, N.Y., Dr. Moon H. Lee has created a rat model for microcephaly, a birth defect in which the rat is born with an abnormally small brain. When this happens in humans, the child is profoundly mentally retarded.

Ten days after birth of the defective rat, Lee implants a section of cortex from a rat fetus, and the transplant will grow, filling in the empty cranium with "fairly large, solid brains."

The new brain "is not exactly normal, but in some ways, it is similar to normal brain," Moon said. It has structure, the nerve cells make connections with each other and try to make connections with the host brain. It fails, however, to make a good connection to the central nervous system, and, although it is electrically active, it does not appear to improve the behavior of the rat, such as learning during memory tests.

Lee said that one day, this approach may help restore the brains of mentally retarded children. "I hope that it will be helpful someday, but I realize that will be far away."